1. Field of the Invention
The field of the invention is that of optoelectronic taxi-assistance devices for aircraft, including a head-up display providing for the presentation of information in the pilot's field of vision. The invention applies in particular to large civil aircraft of the Boding 747 or Airbus A380 type.
2. Description of the Prior Art
It is important that the taxiing phases of aircraft in an airport are able to take place in complete safety regardless of the air traffic density or the visibility conditions.
On modern aircraft, to provide the airport navigation function, there are two types of navigation aid systems. These are:                a navigation aid system comprising in particular an instrument panel display, called a head-down display, presenting an electronic map of the airport, of the position of the aircraft in the airport and of the path to be followed, and including taxi clearance management;        a taxi-assistance system comprising in particular a head-up display presenting superimposed on the external landscape a symbology giving information on the path to be followed and a certain number of instructions, said symbology being generated by a computer dedicated to the head-up display.        
In the case of the taxi-assistance system, the information is supplied to the computer 2 of the head-up display 1, as shown in FIG. 1:                by the main navigation system 3, in particular for heading, ground speed and position information;        by the computer 4 dedicated to the airport navigation function, in particular for information relating to the path to be followed, to stop points, to taxiway changes, to the distance to be covered or to the width of the taxiway.        
The computer combines the specific calculation functions from the information supplied to it and generates the symbology required for the head-up display.
A head-up display comprises an image source, collimating optics and an optical combiner placed in the field of vision of the pilot 5; the head-up display thus gives a virtual image at infinity of the symbology superimposed on the external landscape.
Conventionally, the symbols displayed in a head-up display are separated into two major categories:                “2D” symbols providing navigation information to the pilot, for example:                    the horizon line;            the estimated time or the distance of the aircraft to the final waypoint;            the change of direction to be performed;            the ground speed of the aircraft;            the magnetic heading;                        “3D” symbols which give a better perception of the surroundings of the aircraft. These symbols are particularly useful in poor visibility conditions, for example during nighttime navigation or when navigating in bad weather conditions. These are substantially symbols representing the taxiway. The virtual image of this symbology provided by the head-up display is superimposed exactly on the real position of the taxiway, the position of the aircraft relative to the taxiway being known exactly by means of the navigation systems.        
By way of non-limiting example, FIG. 2 represents a symbology of this type as is presented to the pilot through the head-up display. It comprises:                the horizon line 10 graduated in magnetic headings (values 31 and 33 in the figure, representing the value of the heading in tens of degrees);        the estimated time or the distance of the aircraft to the final waypoint 11 symbolized by the text “HOLD” followed by a number of minutes indicating to the pilot the number of minutes remaining until the final waypoint (2 minutes in FIG. 2). The time indication can be replaced by a distance indication generally expressed in meters;        the change of direction 12 symbolized by the text “TURN” followed by an indication of the number of meters to be covered before the next turn of the aircraft (91 meters in FIG. 2);        the magnetic heading indication 13 symbolized by the value of the magnetic heading surrounded by a rectangle terminated by a V-shaped point arranged on the horizon line 10 (value of the heading in FIG. 2: 322 degrees);        the ground speed 14 symbolized by the text GS followed by the value of the speed expressed in knots, the acronym GS standing for “Ground Speed”. When the speed is excessive (as in FIG. 2), the symbol is drawn in reverse video (in white on a black background in FIG. 2);        the symbols corresponding to the taxiway, their image through the head-up display being superimposed perfectly with the real taxiway. These symbols comprise:                    blocks 15 arranged at regular intervals and symbolizing the right and left limits of the taxiway. In FIG. 2, these blocks are circular but, naturally, appear elliptical due to the perspective effect, their size decreasing with their distance to the aircraft;            rectangles 16 arranged at regular intervals and symbolizing the middle of the taxiway. Naturally, these rectangles are also represented in perspective, their orientation and their size depending on their position relative to the aircraft.                        
The main drawback of this type of symbology is that the information supplied does not present specific symbologies intended to make safe the taxiing of the aircraft on the taxiways. This problem is particularly significant for large capacity civil airplanes which, since they have large dimensions, must be maneuvered with greater precision than smaller airplanes. The location of the pilot, placed very high above the ground, also necessitates adapted symbologies. For example, on an airbus A380, the pilot is placed more than seven meters above the ground.